1. Field of the Invention
The present invention relates to valve seats for internal combustion engines. The present invention particularly relates to a valve seat formed by an iron-based sintered alloy, having high abrasion resistance and also relates to a method for manufacturing such a valve seat.
2. Description of the Related Art
Valve seats, which are press-fitted into cylinder heads of engines, have been used for preventing a combustion gas from leaking and used for cooling valves. Such valve seats must have high heat resistance, abrasion resistance, and corrosion resistance and also have low opposite aggressibility so as to avoid wearing valves, which are opposite members.
For automobile engines, demands have been recently made for improvement in life, power, emission gas, fuel efficiency, and the like. Therefore, it is necessary that the valve seats for such automobile engines can be used in more severe environments than ever. Thus, the valve seats must be further improved in heat resistance and abrasion resistance.
In order to meet such requirements, Japanese Unexamined Patent Application Publication No. 2000-54087 (hereinafter referred to as Patent Document 1) discloses the following material for value seats: an iron-based sintered alloy material, containing Cr—Mo—Si—Co alloy particles dispersed as hard particles in the matrix in the amount of 10–30% on an area basis, and its porosity being 1–10% on a volume basis. A method for manufacturing the iron-based sintered alloy material includes a forming step for filling raw material powder into a metal mold to compact the packed powder to form a green compact, a first sintering step for heating the green compact at 900–1,200° C. in a protective atmosphere to obtain a primary sintered body, a re-pressing/forging step for re-pressing the primary sintered body to obtain a re-pressed compact or forging the primary sintered body to obtain a forged compact, and a second sintering step for heating the re-pressed compact or forged compact at 1,000–1,200° C. in a protective atmosphere. According to the technique disclosed in Patent Document 1, a high-density sintered product, which is an iron-based sintered alloy material having improved high-temperature strength and thermal conductivity, can be obtained.
Japanese Unexamined Patent Application Publication No. 2000-160307 (hereinafter referred to as Patent Document 2) discloses a method for manufacturing a powder metallurgical component, which is fit for a valve seat insert. This method includes a step for press-molding a mixed powder to form a green compact having substantially a reticulated shape and a step for sintering the green compact. The mixed powder contains 15–30% of a valve steel powder, 0–10% of Ni powder, 0–5% of Cu powder, 5–15% of a ferroalloy powder, 0–15% of a tool steel powder, 0.5–5% of a solid lubricant, 0.5–2.0% of graphite, and 0.3–1.0% of a primary lubricant on a mass basis, the remainder being substantially a low-alloy steel powder. The untreated compact has a density of 6.7–7.0 g/cm3, preferably 6.8–7.0 g/cm3, and most preferably 6.9 g/cm3. According to the technique disclosed in Patent Document 2, the powder metallurgical component having a relatively high density can be obtained by a sintering process including a single pressing step. The component further has high abrasion resistance, heat resistance, creep strength, fatigue strength, corrosion resistance, and machinability.
Japanese Examined Patent Application Publication No. 61-10644 (hereinafter referred to as Patent Document 3) discloses a sintered alloy valve seat, monolithically formed by a sintering process, having a double layer structure consisting of a surface layer portion and a base layer portion. The surface layer portion includes a working face repeatedly knocked by a valve face, and the base layer portion is in contact with the bottom of a press-fitting hole of a cylinder head. The surface layer portion has a porosity of 5–20% and the base layer portion has a porosity of 5% or less. The sintered alloy valve seat is fit for a cast-iron cylinder head.
In the technique disclosed in Patent Document 1, the step for re-pressing or forging the primary sintered body and the secondary sintering step are necessary to obtain the high-density sintered compact having a porosity of 1–10%. Therefore, there is a problem in that the manufacturing process is complicated and manufacturing cost is high. In the technique disclosed in Patent Document 3, a step of subjecting the sintered compact to compression-forging by a rotary forging process and a step of re-sintering the resulting sintered compact are necessary to decrease the porosity of the base layer portion; hence, there is a problem in that the manufacturing process is complicated and manufacturing cost is high.
On the other hand, in the technique disclosed in Patent Document 2, the powder metallurgical component having a relatively high density can be obtained by the method including a single molding step and a single sintering step; however, a step for increasing the density is complicated. Therefore, there is a problem in that manufacturing cost is high.
In recent years, for gasoline engines, demands for high power have been growing. Therefore, during the operation of the engines, thermal loads applied to valve seats are greatly increased and impact loads applied to the valve seats by valves are also greatly increased.
Under such conditions, adhesive wear readily occurs on the surfaces of valves and valve seats; hence, fresh surfaces functioning as sliding surfaces are repeatedly formed. Therefore, there is a problem in that the valves and valve seats are seriously worn.